The importance of the Notch receptor system in central nervous system (CNS) demyelinating disease has been suggested by experiments where inhibition of -secretase and antibody blocking of Delta-Like Ligand (DLL) 1 down-regulated inflammatory T cell cytokine responses. However, we were the first to recently demonstrate that blockade of DLL4 function inhibited the development of clinical experimental autoimmune encephalomyelitis (EAE). Our data suggested that the role of DLL4 was to regulate encephalitogenic T cell chemokine receptor expression and subsequent trafficking to the CNS and not regulation of either Th1 or Th17 responses although DLL4 has been reported to have that function. DLL4 appears to be a significant molecule involved in the pathogenesis of EAE as it has a greater ability than DLL1 or Jagged 1 to regulate T cell activation. That finding together with our preliminary data indicating that T cell activation with anti-CD3, anti-CD28 and recombinant DLL4, but not recombinant DLL1 or Jagged1, up-regulated chemokine receptors provides strong rationale to pursue DLL4- mediated Notch regulation of T cell migration in EAE as a critical element in the pathogenesis of disease. Therefore, our overall hypothesis is that DLL4 regulates EAE development by modulating T cell effector trafficking mechanisms. Three specific aims are proposed to directly test this idea, determine the mechanism behind the original observation and translate these findings into a novel therapy for multiple sclerosis (MS). In specific aim 1 we will determine the mechanism behind DLL4 regulation of T cell chemokine receptor expression. In specific aim 2 we will determine whether endothelial DLL4 engagement with T cell Notch cells to regulate T cell chemokine receptor expression. In specific aim 3 we will determine anti-DLL4 treatment efficacy and the role of DLL4 in epitope spreading as a mechanism of relapsing EAE. The proposed work therefore has high biomedical significance and will introduce conceptual and technical innovations to the area of MS research.